FOIA declassified documents:

The famous "toilet seats" letter from the FBI:

This document is the very famous letter from Edgar J. Hoover, director of the FBI, to General George McDonald. It expresses the anger of the FBI when they were asked by the US Army Air Force to let them study serious flying saucer cases, while the FBI should only investigate cases of hoaxes and trivial explanable events. General McDonald did not respond to Mr. Hoover's letter.

Interest:

This document shows that in 1947, the FBI knew that the US Army Air Force considered that there were "serious" flying saucers besides hoaxes and jokes that should be investigated by the military intelligence only.